Process for manufacturing polyamides

ABSTRACT

The invention relates to a continuous process for manufacturing polyamides. The polyamides are of the type obtained from diacids and diamines. The process comprises an operation of continuous mixing of a compound which is rich in amine end groups and a compound which is rich in acid end groups and a polycondensation operation using the mixture. The invention relates to the starting phase of such a process, during which an aqueous solution comprising a mixture of monomers in substantially stoichiometric proportions is used.

[0001] The present invention relates to a continuous process formanufacturing polyamides. The polyamides are of the type obtained fromdiacids and diamines. The invention relates more particularly to thestarting phase of processes for manufacturing polyamides.

[0002] Polyamides are polymers of major industrial and commercialinterest. Thermoplastic polyamides are obtained either by reactionbetween two different monomers or by polycondensation of a singlemonomer. The invention applies to polyamides obtained from two differentmonomers, the most important polyamide of which is poly(hexamethyleneadipamide). Obviously, these polyamides may be obtained from a mixtureof diacids and diamines. Thus, in the case of poly(hexamethyleneadipamide), the main monomers are hexamethylenediamine and adipic acid.However, these monomers may comprise up to 25 mol % of other diamine ordiacid monomers or even amino acid or lactam monomers.

[0003] This category of polyamides derived from two different monomersis generally manufactured using, as starting material, an amino acidsalt obtained by mixing a diacid with a diamine in stoichiometric amountand in a solvent such as water.

[0004] Thus, in the manufacture of poly(hexamethylene adipamide), adipicacid is mixed with hexamethylenediamine in water to obtain ahexamethylenediamine adipate, which is more commonly known as Nylon saltor “N salt”.

[0005] The solution of N salt is optionally concentrated by evaporatingoff the water.

[0006] The polyamide is obtained by heating such a solution of Nylonsalt at high temperature and pressure to evaporate off the water, whileat the same time avoiding any formation of solid phase to avoid themixture setting to a solid.

[0007] This operation consumes a large amount of energy, and also doesnot make it possible to control the stoichiometry entirely since theamine may be evaporated or entrained with the water. Such an entrainmentor evaporation requires control of the process to re-establish thestoichiometry, and may be an inconvenience since the diamine entrainedmay contaminate the effluents discarded from the production plant.

[0008] In addition, the need to heat to a high temperature under a highpressure may cause the formation of degradation compounds whichadversely affect the performance qualities of the manufacturedpolyamide, especially as regards the mechanical characteristics and itscolour.

[0009] To avoid the use of large amounts of water, certain processes forpreparing a polyamide without water and without solvent have alreadybeen proposed. Mention is made of documents WO 96/16107, WO 99/61511 andWO 00/77075.

[0010] Processes described in these documents comprise the followingsteps:

[0011] continuous mixing of two flows of polyamide precursors, one flowrich in acid end groups, and one flow rich in amine end groups,

[0012] condensation using the mixture of the two flows,

[0013]  the mixing and condensation being carried out in the same plantor in different plants,

[0014] recovery of a flow of product obtained from the condensationoperation,

[0015]  at least one of the flow rates of the flows of precursor beingcoupled to the result of the analysis of the contents of acid and amineend groups in the mixture, the condensation medium or the recoveredflow, so as to keep these contents and/or their ratio between twonominal values.

[0016] More specifically, document WO 99/61511 describes a process inwhich adipic acid (AdOH) melt is mixed with hexamethylenediamine (HMD)in liquid form or as a highly concentrated solution. The mixture isintroduced into a recycling reactor fitted with heating means. Themonomer flows are coupled to the analysis of the amount of acid endgroups and amine end groups in the compound obtained. One analysismethod described is Near InfraRed (NIR) spectroscopy. The starting phaseof the process, that is to say the operations preceding theestablishment of a permanent regime, is not described.

[0017] Document WO 00/77075 mentioned above describes the mixing of twoflows of desequilibrated mixtures M2 and M3, each respectivelycomprising more and less of diacid than of diamine, in diacid/diamineratios of between 1.005 and 1.2 and between 0.8 and 0.995, respectively.One analysis method described is also Near InfraRed (NIR) spectroscopy.The mixtures M2 and M3 may be obtained, respectively, by mixing adiacid/diamine mixture, with a composition in the region of 81:19 bymass and of a diamine: this diamine is introduced, respectively, indeficit and in excess relative to the amount of diacid in the mixturesM2 and M3. The starting phase of the process is not described.

[0018] The performance qualities of the processes described above,especially as regards the quality of the polyamide and the productionefficiency, are linked to their regulation, by metering the productobtained and coupling the flows of precursors to this monitoring. Aslong as the product recovered does not comply with the specifications,the material used is considered as being lost. This has a greatinfluence on the viability of the process. The starting phase, as far asthe production of a satisfactory regulation and permanent regime, withan exiting product which complies with specifications, is thus of greatimportance. In addition, the further the product obtained from thestarting phase is removed from the point of functioning, the longerand/or more difficult the regulation is to obtain. This is particularlytrue in the context of monitoring by NIR spectrometry, which issensitive, requiring a prior calibration by physical, chemical orphysicochemical analysis methods. Regulation using this method, whichmoreover has other advantages, for obtaining a point of functioningwhich is remote from the starting point, may prove to be long.

[0019] The observation cannot be avoided that the prior art does notcontain any teaching regarding the starting procedures for processes forthe continuous manufacture of polyamide using a mixture of two flows ofprecursors, one rich in diacids and the other rich in diamines.

[0020] The object of the present invention is to propose a process formanufacturing polyamide, comprising a starting phase that isparticularly advantageous in many respects. It makes it possible toproduce tailings in plants, preventing phenomena of precipitation,crystallization, degradation or unwanted heterogeneity; it makes itpossible to achieve a rapid and satisfactory regulation; it makes itpossible to avoid large losses of material, the product then leaving thestarting phase being a polyamide obtained according to a process relatedto a common process starting with an aqueous solution of astoichiometric diacid/diamine salt.

[0021] To this end, the invention proposes a process for the continuousmanufacture of polyamide, using dicarboxylic acids and diamines,comprising the steps described above in a permanent regime, and thestarting phase of which comprises the following steps:

[0022] Step 1:

[0023] feeding the condensation plant with an aqueous solutioncomprising dicarboxylate ions and diammonium ions, in proportions suchthat their molar ratio is between 0.99 and 1.01 and preferably between0.995 and 1.005,

[0024] establishment of the operating regime in the condensation plantunder temperature and pressure conditions such that there iscondensation starting with the dicarboxylate ions and the diammoniumions initially included in the aqueous solution,

[0025] Step 2: recovery of a flow of the condensation product

[0026] Step 3: supplying a flow of precursors and activating theanalysis and the coupling.

[0027] The polyamide is of the type obtained from monomers ofdicarboxylic acid type and from monomers of diamine type.

[0028] The expression “precursor rich in acid end groups” may beunderstood as meaning:

[0029] monomers of the pure diacid type (that is to say free of monomersof diamine type), in solid or melt form, or as a concentrated solution(optionally in ionized form),

[0030] mixtures of monomers of diacid type and of monomers of diaminetype, in non-stoichiometric proportion, comprising more monomers ofdiacid type than monomers of diamine type, in melt form and/or in theform of a concentrated solution (optionally in ionized form),

[0031] amidation products, obtained from mixtures of monomers of diacidtype and of monomers of diamine type, with an amount of acid end groupswhich is greater than the amount of amine end groups, preferably with adegree of polymerization of less than 20.

[0032] The expression “precursor rich in amine end groups” may beunderstood as meaning:

[0033] monomers of pure diamine type (that is to say free of monomers ofdiacid type), in solid or melt form, or in the form of a concentratedsolution (optionally in ionized form),

[0034] mixtures of monomers of diacid type and of monomers of diaminetype, in non-stoichiometric proportion, comprising more monomers ofdiamine type than monomers of diacid type, in melt form and/or in theform of a concentrated solution (optionally in ionized form),

[0035] amidation products, obtained from mixtures of monomers of diacidtype and of monomers of diamine type, with an amount of amine end groupswhich is greater than the amount of acid end groups, preferably with adegree of polymerization of less than 20.

[0036] In a permanent regime, the process comprises at least oneoperation of mixing a flow of a precursor which is rich in diacid endgroups, and a flow of a precursor which is rich in diamine end groups.The mixing operation may be carried out in a plant specially designedfor this purposes, for example in a static mixer zone. It may be carriedout in a plant in which other operations are also carried out, forexample a condensation operation. It may be, for example, a stirredreactor or a recycling reactor in which the two flows are simultaneouslyintroduced. The plant in which the mixing is carried out is referred toas the mixing plant.

[0037] The mixture is subjected to a condensation operation, t: at is tosay an operation during which monomers or oligomers react to formmacromolecular chains, or polyamide oligomers. During this operation,the degree of polycondensation increases. The condensation operation isgenerally carried out at temperatures above 180° C. and at a pressureabove 5 bar, preferably at temperatures above 220° C. and at a pressureof between 5 and 20 bar. The pressure indications of the presentinvention are absolute pressures, opposed to relative pressures. Thecondensation product preferably has a degree of polycondensation ofgreater than 10, preferably of greater than 20. The product obtainedfrom this operation may be referred to as a prepolymer, if its degree ofpolycondensation does not exceed 100. The plant, or plants, in which thecondensation is carried out is(are) referred to as the condensationplant(s). If the plants in which the mixing and condensation areperformed are the same, the said plant is referred to as the mixingplant or condensation plant depending on the operation underconsideration.

[0038] A flow of the product obtained from the condensation operation isrecovered. In a permanent regime, the flow rate of material obtainedfrom this flow is substantially equal, in terms of amount of material,to the sum of the flow rates of the flows of precursors. The amounts ofacid and amine end groups in the product constituting this flow aremeasured, and at least one of the flow rates of the flows of precursorsare coupled to this measurement, such that these amounts remain betweentwo nominal values, and/or are such that the ratio between the amount ofacid end groups and the amount of amine end groups is between twonominal values, preferably close to 1. The analysis may also be carriedout on the mixture of the flows, or on a product duringpolycondensation. As a guide, the preferred margin of variation of thismolar ratio compared with the desired value is plus or minus 0.0005.

[0039] One particularly advantageous analysis method for carrying outthe process is Near InfraRed spectrometry. This technique fordetermining a property of a polymer has been described, for example, inU.S. Pat. No. 5,532,487 and WO 96/16107. Thus, in U.S. Pat. No.5,532,487, the method of spectrometric analysis by Near InfraRed is usedto determine the concentrations of the acid and amine end functions in apolyamide in solid form, for example on a yarn, or on anhydrousdiacid/diamine mixtures.

[0040] Similarly, patent WO 96/16107 describes the use of a NearInfraRed spectrometric analysis method to determine the concentration ofacid and/or amine end functions in a polyamide melt leaving a reactor.However, in these two examples, the polyamide analysed is substantiallyanhydrous.

[0041] In one particular embodiment of the invention, this determinationof the concentration of acid and/or amine end functions is performed byanalysis of the reaction mass containing the water resulting from theamidation reaction, for example in a branch loop of the main flow, in anoptional branch of the main flow in a recycling loop of a reactor.

[0042] Depending on the degree of polycondensation of the productrecovered, this product may be introduced into flashing and/or finishingplants, in order further to increase the said degree ofpolycondensation. The flashing operation is intended to return theproduct to atmospheric pressure. It is generally followed by aseparation of the water produced during the condensation. The separationmay be carried out in a plant which is open to gases, for example aseparator of cyclone type. The successive flashing and finishing stepscomprise a rapid evaporation of the condensation water contained in thepolyamide leaving the polymerization reactor, obtained, for example, bydepressurizing the flow of polyamide. The polyamide is then maintainedfor a given period at a polymerization temperature under atmosphericpressure, or under reduced pressure, to obtain the desired degree ofpolycondensation.

[0043] The plants may furthermore comprise means for conveying the flowsof materials, or for placing them under the temperature and pressureconditions mentioned, such as pumps, valves, heat exchangers, pressuremonitoring and regulating devices, analysis instrumentation, etc.

[0044] In a first starting step of the process, the condensation plant,and optionally the mixing plant, are fed with an aqueous solutioncomprising dicarboxylate ions and diammonium ions, in molar proportionsof between 0.99 and 1.01 and preferably between 0.995 and 1.005. Thisratio is very close to the stoichiometry, and the solution is notconsidered as a precursor which is rich in acid or amine end groups. Itmay be, for example, an aqueous hexamethylenediammonium adipatesolution, at a weight concentration of between 40% and 65%. This firstfeed is carried out so as to fill the plants at least partially. Valvesfor cutting the recovery of the products before the condensation plantsmay be closed during this first feed. The feed may comprise severalphases, for example a phase of preliminary feeding with water, followedby a phase of feeding with a solution comprising ions, or with a salt insolid form, or with acid and amine monomers, in order to obtain thesolution. The stoichiometry of the dicarboxylate and diammonium ions maybe controlled, for example, by pH measurement or by Near InfraRedspectrometry.

[0045] After feeding in the solution, the operating temperature andpressure are established in the condensation plant such that there iscondensation starting with the ions initially included in the solution.

[0046] In the embodiment for which the acid monomer is adipic acid andthe amine monomer is hexamethylenediamine, the solution comprising thehexamethylenediammonium salt is preferably heated to a temperature above180° C. and to a pressure above 5 bar. This operation is advantageouslycarried out in a reactor under pressure. The plants may comprise devicesspecially intended for carrying out this phase, such as devices forremoving the water, and/or for working under pressure. Such conditions,depending on the types of plant, are known to those skilled in the art.

[0047] During a second step, a flow of product obtained from thecondensation is recovered from the solution. According to oneadvantageous mode, the product recovered has contents of acid and amineend groups that are very close to those intended in a permanent regime.

[0048] It is pointed out that the phases of feeding with solution and ofcondensation from the solution may be carried out on continuous flows.The flows of solution or of condensation product still not correspondingto the desired level of condensation may be removed, stored orreintroduced into the plants. These flows do not correspond to the flowsrecovered from the condensation product.

[0049] During a third step, the operations allowing the process tofunction in a permanent regime are carried out. Thus, the plants arefed, optionally via a mixing plant, with a flow of precursors, and thesystem for analysing and coupling at least one of the flow rates ofthese flows is activated. The feeding with a flow of precursors and theactivation of the analysis and coupling may be successive orsimultaneous. The analysis is advantageously carried out by NearInfraRed spectrometry. According to one advantageous process, at leastsome of the flow of condensation product from the solution is recoveredand used to calibrate the Near InfraRed spectrometer in the desiredrange of contents of end groups and/or of ratios of contents, thefeeding with two flows, the analysis and the coupling being activatedafter this operation. To this end, the method of dosed additions may becarried out: known amounts of diacid and/or of diamine are added to theproduct and the spectrometer is calibrated on the response obtained.

[0050] The feeding with a flow of precursor may be understood as thefeeding of a precursor prepared beforehand in another plant. It may alsobe understood as the activation of a system of analysis of a flow and ofcoupling parameters for preparing the flow to this analysis. The flowfed in may thus change gradually from a flow of product which is notconsidered as the precursor in permanent regime to a flow of productconstituting the precursor.

[0051] In the course of time, the product recovered changes graduallyfrom a product obtained by polycondensation mainly from the aqueoussolution to a product obtained by polycondensation mainly from themixture of the two flows of precursors. This process makes it possiblerapidly to achieve a regulation of the process in a permanent regime,without any loss of product, the product recovered at the start of theprocess relating to a polyamide obtained from a salt solution, insubstantially stoichiometric proportions.

[0052] It is pointed out that the second and third steps may beperformed simultaneously or successively.

[0053] As mentioned above, in the course of the second step, there is agradual change from a condensation starting with an aqueous solution ofsalt to a condensation starting with a mixture of precursors. Thecondensation conditions may vary substantially during this step.According to one preferential embodiment, the condensation temperatureand pressure conditions for step 1 and for the permanent regime areidentical.

[0054] The process of the invention may be used for the manufacture ofpoly(hexamethylene adipamide), starting with adipic acid as diacidmonomer and hexamethylenediamine as diamine monomer.

[0055] The process of the invention also makes it possible tomanufacture other polyamides starting with diacid monomers chosen fromthe group comprising glutaric acid, suberic acid, sebacic acid,dodecanedioic acid, isophthalic acid, terephthalic acid, azelaic acid,pimelic acid or naphthalenedicarboxylic acid, for example.

[0056] Diamine monomers which may be mentioned, in addition tohexamethylenediamine, include heptamethylenediamine,tetramethylenediamine, octamethylenediamine, nonamethylenediamine,decamethylenediamine, 2-methylpentamethylenediamine,undecamethylenediamine, dodecamethylenediamine, xylylenediamine andisophoronediamine.

[0057] The invention preferentially applies to the manufacture ofpolyamide from diacid and diamine monomers, at least 80 mol % of whichare, respectively, adipic acid and hexamethylenediamine.

[0058] It is also possible to prepare polyamides from diacid and diaminemonomers comprising a small proportion (less than 20 mol %) of aminoacid or of lactam. The manufacture of copolyamide PA 6,6/6 may thus bementioned, starting with adipic acid/hexamethylenediamine monomerscontaining a given amount of caprolactam.

[0059] According to a first embodiment of the invention, the precursorwhich is rich in acid end groups is a diacid and diamine melt, with adiacid/diamine molar ratio of between 0.8 and 0.995 and preferablybetween 0.8 and 0.99; the precursor which is rich in amine end groups isa diacid and diamine melt, with a diacid/diamine molar ratio of between1.005 and 1.2 and preferably between 1.01 and 1.2. The condensationplants preferably comprise a stirred polymerization reactor.

[0060] The functioning of the process in a permanent regime is firstdescribed.

[0061] The diacid/diamine molar ratio of the precursor which is rich indiacid end groups is preferably between 0.95 and 0.99. Thediacid/diamine molar ratio of the precursor which is rich in amine endgroups is preferably between 1.01 and 1.05. The plants may compriseflashing and finishing devices downstream of the polymerization reactor.

[0062] Preferably, the mixtures constituting the precursors areanhydrous. The expression “anhydrous mixture” should be understood asmeaning mixtures which may contain up to 10% by weight of water. Theterm “anhydrous” is used in the present description as opposed to theconventional process which uses an aqueous solution of Nylon salt inpermanent regime.

[0063] The mixtures constituting the two flows of precursors may beprepared in reactors, at a temperature advantageously between 100 and260° C. under a pressure at least slightly above atmospheric pressure,preferably at a temperature of between 220° C. and 260° C. at a pressureof between 5 and 20 bar. The reactors in which the precursors areprepared and also the polymerization reactor may be, for example,reactors with mechanical stirring or reactors with external recycling.

[0064] In the latter case, the feeding with precursors and/or theremoval of the product and/or the heating may advantageously beperformed in the recycling loop. The heating of the reactors may beperformed using a jacketed device and/or an internal coil. The reactorsmay furthermore be open to gases.

[0065] According to one particular mode, the precursors are prepared bymixing, in the liquid state, a diacid with the diamine, in the presenceof a small amount of water, with heating at moderate temperature for aprecursor which is rich in diacid or diamine end groups.

[0066] According to another particular mode, the precursors are heatedat a higher temperature with removal of water to obtain condensationproducts, oligomers or prepolymers, containing acid end groups for thefirst precursor and amine end groups for the second precursor. For themanufacture of polyhexamethylene adipamide, the temperature isadvantageously between 180° C. and 250° C., under a pressure of between5 and 20 bar. The degree of polycondensation of the precursors ispreferably less than 20. The precursors are then mixed together andintroduced into the polymerization reactor, in which the degree ofpolymerization is increased.

[0067] According to one preferred characteristic, the condensationperformed in the polymerization reactor is carried out under anautogenous pressure or a pressure regulated to avoid any loss of diamineor at least to reduce the losses to the minimum. According to anothercharacteristic, the plants may comprise means for recovering any diaminevolatilized at the same time as the water, and means for introducing thediamine recovered into the polymerization plant.

[0068] According to a novel characteristic of the invention, thepolymerization performed in the polymerization reactor is carried outunder an autogenous pressure or a pressure regulated to avoid any lossof diamine or at least to reduce the losses to the minimum.

[0069] The process may be used to manufacture poly(hexamethyleneadipamide), starting with adipic acid as diacid monomer andhexamethylenediamine as diamine monomer.

[0070] In one preferential embodiment, poly(hexamethylene adipic)(polyamide 6,6) is manufactured, and the precursors are prepared from anadipic acid (AdOH)/hexamethylenediamine (HMD) mixture with a compositionin the region of a eutectic mixture, that is to say 19% by weight of HMDand 81% by weight of AdOH. This eutectic mixture has a melting point inthe region of 100° C.

[0071] The eutectic mixture is introduced into two reactors into whichare introduced hexamethylenediamine to obtain, respectively, a firstmixture which is rich in diacid (diacid/diamine ratio of between 1.01and 1.2) and a second mixture which is rich in diamine (diacid/diamineratio of between 0.8 and 0.99). The process for preparing the eutecticmixture may be analogous to that described in U.S. Pat. No. 4,131,712for the preparation of acid-rich mixtures.

[0072] The eutectic mixture is advantageously prepared in plantsupstream of the plants for preparing the precursors. This may be, forexample, a plant equipped with stirring and heating means, which may beplaced under pressure. Such a plant for preparing the eutectic mixturemay be used to feed the two plants (generally reactors) for preparingthe precursors.

[0073] Advantageously, the diacid/diamine ratios of the precursors aremonitored by chemical or potentiometric analyses. In one particularlypreferred embodiment, these diacid/diamine ratios are determined by aNear InfraRed spectrometric analysis.

[0074] The eutectic mixture may be prepared by mixing adipic acid, insolid or melt form, and hexamethylenediamine, in the pure form or in theform of a concentrated solution, the mixing being performed attemperatures above 100° C., under a pressure which is at least slightlyabove atmospheric pressure.

[0075] The temperature for preparing the precursors is advantageouslyabove 100° C. to obtain an amidation reaction and thus a partiallycondensed product, containing acid or amine end groups depending on theprecursor.

[0076] The flows of precursors are collected and introduced into astirred polymerization reactor. It is possible to include static mixersin the feed pipe of the polymerization reactor and/or a premix reactor.

[0077] The flow rate of each flow of precursor is determined so as toobtain a ratio between the amine and acid functions which is as close aspossible to the desired value. The flow rate of the flows is monitoredand adjusted by coupling it to the result of the analysis of the ratiobetween the acid and amine functions present either in the reaction massformed by the mixing of the two flows, for example in the premix reactoror after the static mixers, or in the reaction mass present in thepolymerization reactor or at the outlet of the said polymerizationreactor.

[0078] The adjustment of the flow rates for entry into the variousreactors may be carried out by means of pumps or depressurizationvalves.

[0079] In order to obtain an efficient adjustment, analysis of the acidand amine functions is performed continuously by Near InfraRedspectrometric analysis. The coupling may also be combined withmeasurement of the stoichiometric ratio in the two flows of precursors.

[0080] The starting steps preceding the functioning in permanent regimeare now described for the first embodiment of the invention describedabove.

[0081] The polymerization reactor and optionally the mixing device, theplants for preparing the precursors and the plants for preparing aeutectic mixture are fed with an aqueous solution comprisingbicarboxylate ions and diammonium ions. This is preferably ahexamethylenediammonium solution with a concentration of between 40% and65% by weight. During the feeding of salt solution, a product recoveryvalve, downstream of the polymerization reactor, is preferably closed.

[0082] During a second step, the salt solution in the polymerizationreactor is heated to a temperature above 100° C., under a pressurepreferably of between 1 and 10 bar.

[0083] The polymerization reactor is fed with a mixture of precursors.In a first variant, the precursors are already prepared, in therespective proportions of the permanent regime. In a second variant, theprecursors are not yet prepared on starting the plants, the plants forpreparing these precursors containing a salt solution. In the secondvariant, the proportions of acid monomers and of amines, respectively,for the precursors in the permanent regime are gradually reached byfeeding diacids or a eutectic mixture and/or diamine into the plants forpreparing the precursors. The precursor mixture then changes graduallyfrom a mixture obtained from salt solutions to a mixture obtained fromprecursors that are rich in acid and amine end groups, respectively.

[0084] The condensation product obtained from the polymerization reactoris recovered and the analysis and coupling system is activated.

[0085] For the second variant, the temperature is advantageously between100 and 180° C., with a pressure of between 1 and 10 bar, such that thesalt solution is liquid. A start of condensation may take place in theseplants.

[0086] When carrying out the second variant, the precursor preparationplants contain a salt solution at the start of the process, thediacid/diamine ratios being substantially equal to 1. These ratios aremodified by feeding into the plants different amounts (different flowrates) of diamine-rich compounds and of diacid-rich compounds, into eachof the preparation plants. As described above, a diacid-rich compoundwhich is preferred for preparing the precursors is a eutectic mixture.

[0087] The eutectic mixture may be prepared in plants upstream of theprecursor preparation plants. The plant is, for example, a tank equippedwith mixing and heating means. According to one preferentialcharacteristic, the plant for preparing the eutectic mixture is firstfed with an aqueous salt solution. The mixture is heated to atemperature of between 100 and 150° C. The plant is fed with diacid, forexample in solid form, and optionally with diamine, with feed ratescorresponding to the stoichiometry of the eutectic mixture, and theproduct obtained is removed. The residence time in the plant ispreferably chosen such that the diacid, if it is in solid form, has thetime to melt and/or dissolve. The temperature in the plant is greaterthan the melting point of the mixture. It is pointed out that themixture may contain water, either added continuously, in an amountpreferably less than 10% by weight, or obtained from the initial feed ofsalt solution. Water obtained from the initial feed of salt solution isremoved either by evaporation, before feeding in diacid, or bydepletion, during the removal of the mixture prepared, on starting theprocess.

[0088] According to a second embodiment of the invention, the precursorwhich is rich in acid end groups is a dicarboxylic acid melt, theprecursor which is rich in amine end groups is a diamine melt or adiamine in concentrated solution. The condensation plants preferablycomprise two reactors, a reactor which is not open to gases and areactor which is open to gases. The plants comprise a mixing plant, forexample a zone of static mixers.

[0089] In a permanent regime, the two flows are mixed together such thatthe amounts of diacids and of diamines are equivalent. The plants maycomprise devices for ensuring that the diacid is not in contact withoxygen. In the context of manufacturing polyamide 6,6, the diacid isadipic acid, preferably introduced at a temperature close to 170° C.,and the diamine is hexamethylenediamine, preferably introduced at atemperature close to 70° C., optionally as a concentrated solution inwater (preferably less than 10% water).

[0090] The condensation temperature in the reactor that is not open togases is preferably between 220° C. and 300° C., under a pressure ofbetween 5 and 20 bar.

[0091] The residence time of the flow of material in the reactor (theflow entering the reactor constituting the mixture) is advantageouslybetween 1 minute and 30 minutes.

[0092] According to one advantageous characteristic, the condensation inthe reactor is such that only 10% free monomers remains in the flowleaving this reactor.

[0093] The flow obtained from the reactor that is not open to gases canthen be introduced into a reactor which is open to gases. The residencetime of the material in this reactor is advantageously between 1 minuteand 60 minutes. The opening of this reactor to gases allows the waterobtained from the condensation reaction to evaporate off. The degree ofpolymerization also increases in this reactor.

[0094] A flow of the condensation product is recovered. According to onepreferential mode, some of the flow produced is reintroduced into thereactor that is not open to gases, at the temperatures and pressuresmentioned above, in the presence of the mixture of precursors. Thisrecycling of the product is preferentially such that the recycling flowis between ten and twenty times greater than the flow of mixtureintroduced into the reactor which is not open to gases. During thisoperation, there is condensation from the salt. A recovery valve isopened. The degree of polycondensation at the time of opening of thisvalve is advantageously greater than 10 and preferably greater than 20.

[0095] This recycling can make it possible to obtain sufficiently largedegrees of polycondensation, to produce stirring and to heat thematerial.

[0096] The flow obtained from the reactor which is open to gases isanalysed in order to determine the amounts of acid and amine end groups.NIR spectrometry is advantageously used to do this. The flows ofprecursors, diacid melt or diamine are coupled to the analysis of theamounts of end groups and/or their ratio. The ratio between the acid endgroups and the amine end groups is preferably close to 1, advantageouslywith a precision of greater than 0.5. The analyses may also be performedon a recycling flow.

[0097] The flow of product recovered may advantageously be introducedinto a flashing device and then a finishing device, making it possiblefurther to increase the degree of polymerization.

[0098] It is pointed out that the reactor which is open to gases may beprovided with means for recovering the diamine monomers which may havevolatilized with the water. These monomers may be reintroduced into theplants.

[0099] The starting steps preceding the permanent regime described aboveare now described.

[0100] The condensation plants, and optionally mixing plants, are firstfed with an aqueous solution of a diacid/diamine salt, preferably anaqueous hexamethylenediammonium solution of from 40% to 65% by weight.This operation constitutes a first step. During a second step, thesolution included in the condensation plant, for example a reactor whichis not open to gases, is heated to a temperature above 180° C. at apressure above 5 bar, preferably to a temperature above 220° C. and to apressure of between 5 and 20 bar. Recycling of the solution isoptionally imposed. The salt solution included in the mixing plant isadvantageously heated to a temperature of 180° C.

[0101] The mixing plant is fed with a flow of precursors, at flow ratessuch that the proportions of monomers introduced are substantiallystoichiometric. The analysis and coupling system is activated. The flowof mixture is fed into the condensation plant, for example the reactorwhich is not open to gases, and a flow of condensation product isrecovered. The material obtained from the mixing of the precursors thusreplaces in the plants the material obtained from the solution of Nsalt. Water is gradually removed either in the reactor which is open togas, or by depletion with the product recovered.

[0102] The permanent regime is thus gradually reached.

[0103] Other advantages or details of the invention will emerge moreclearly in the light of the examples given below, purely as a guide, andwith the detailed descriptions given with reference to the figures,which represent a synoptic scheme of parts of the process according tothe invention.

EXAMPLE 1 Example of Functioning of the Process and its Start-Up

[0104] The process in permanent regime is first described in thisexample.

[0105] According to the scheme represented in FIG. 1, a concentratedaqueous hexamethylenediamine solution with a concentration by mass ofwater equal to 10%, available from a source 151, and of adipic acidpowder, available from a source 152, is fed continuously into a firststirred reactor 101 via the pipes 110 and 111, respectively, to obtain amixture whose weight ratio is 81% of diacid monomer and 19% of diaminemonomer. This mixture M1 may contain a small amount of water, forexample about 7% by weight relative to the diacid monomer/diaminemonomer mixture. The mixture is maintained at a temperature of about126° C.

[0106] The mixture M1 is fed by withdrawal from the reactor 101 via thepipes 102 and 103, respectively, into two stirred reactors 104 and 105.

[0107] However, in an embodiment which is not shown, the mixture Ml fromthe reactor 101 may be fed into a storage tank and then from there fedinto the reactors 104 and 105 to allow a more flexible execution of theprocess.

[0108] In the example illustrated, the reactor 104 is maintained at 228°C. under a pressure of 15 bar, the flow of mixture M1 fed via the pipe102 is 41 kg/h. Into this reactor 104 is fed a solution ofhexamethylenediamine (HMD) containing 10% water via the pipe 112 at aflow rate which is controlled to obtain in the reactor 104 a mixture M2comprising a ratio of the acid functions relative to the amine functionsequal to 1.03.

[0109] The acid/amine ratio is measured continuously in the reactor 104or, as illustrated, at the outlet of this reactor by a Near InfraRedanalysis method described below. The result of this analysis isprocessed by a coupling system which controls the flow rates of theflows of mixture Ml and of HMD solution fed into the reactor 104.

[0110] In FIG. 1, the dashed lines represent, on the one hand, thecouplings of the flows to the Near InfraRed measurements, and, on theother hand, the analyses of the compositions of the flows by NearInfraRed.

[0111] The residence time of the mixture in the reactor 104 is about 48minutes. The reactor 104 comprises an outlet 106 for removing the watercontained and/or formed in the reactor. The rate of removal of thiswater in the form of steam is 7.6 kg/h.

[0112] The mixture M2 removed from the reactor 104 is a preamidatedadipic acid/HMD mixture which is rich in acid functions.

[0113] A second preamidated adipic acid/HMD mixture M3 is produced inthe reactor 105, in a similar manner to the production of the mixture M2in the reactor 104. However, the flow rate of the flows of mixture M1and of hexamethylenediamine solution fed via the pipe 113 are determinedand monitored so as to obtain in the reactor 105 a mixture with a ratioof the acid functions relative to the amine functions equal to 0.98.

[0114] In the example illustrated, the temperature and pressureconditions are identical to those in the reactor 104.

[0115] As for the manufacture of the mixture M2, the flow rates of theflows of mixture M1 and of HMD are coupled to the result of thecontinuous measurement of the acid function/amine function ratio in themixture M3, by a Near InfraRed analysis method described below.

[0116] The two mixtures M2 and M3 obtained from the reactors 104 and105, respectively, are introduced into a polycondensation reactor 107,maintained at a temperature of 248° C. under a pressure of 17.5 barabsolute.

[0117] The two flows of mixture M2 and M3 are fed into a premixer 108consisting, in the example, of an assembly of static mixers arranged ina pipe. Other mixing means, such as a stirred tank, may be used.

[0118] The flow rates of the flows of mixtures M2 and M3 are monitoredso as to obtain in the reactor 107 a mixture with a given acidfunction/amine function ratio depending on the characteristics of thepolyamide to be manufactured.

[0119] Thus, in the example illustrated, these flow rates are determinedso as to obtain a mixture in the reactor 107 which has a differencebetween the concentrations of acid and amine functions (CEG-AEG) at theoutlet of the reactor 107 in the region of 50 meq/kg.

[0120] This difference in concentrations, or the acid function/aminefunction ratio, is measured continuously by means of a Near InfraRedspectrometric analysis method, the flow rates of the flows of mixture M2and M3 fed into the premixer 108 being coupled to this measurement so asto maintain the difference value between two nominal values.

[0121] The residence time of the mixture or prepolymer in the reactor107 is in the region of 30 minutes.

[0122] The reactors 104, 105 and 107 are equipped with adepressurization valve 106, 109 to allow some of the water formed by theamidation reaction to be removed. The flow of steam removed via thedepressurization valve 109 is equal to 4.5 kg/h on average. The analysisand monitoring of the composition of the flow 114 removed from thereactor 107 allows the progress of the amidation reaction and thus thedegree of polycondensation of the prepolymer removed from the reactor107 to be monitored.

[0123] The average rate of removal of the prepolymer from the reactor107 via the pipe 114 is 102 kg/h. The prepolymer thus recovered has anumber-average molar mass in the region of 3 800 and contains about 5%water. Its viscosity index measured in dilute formic acid in accordancewith the conditions and recommendations of the international standardISO 307-1984 is 33.5 ml/g.

[0124] The process illustrated makes it possible to produce continuouslya prepolymer 153 at the outlet of the reactor 107 with a concentrationof amine end groups (AEG) equal on average to 238.2 meq/kg, and aconcentration of acid end groups (CEG) equal on average to 289.5 meq/kg,that is to say a difference between the concentrations of acid functionand of amine function of 51.3 meq/kg, for a nominal and desired value of50 meq/kg.

[0125] The prepolymer thus produced is converted into polyamide of thedesired molar mass which is compatible with the usual uses by additionto a flow for manufacturing a polyamide obtained from a process formanufacturing polyamide 6,6 starting with hexamethylenediamine adipatesalt.

[0126] The continuous and usual processes for manufacturing PA 6,6described, for example, in the book “Polymerization processes”Schildknecht edition (Wiley Interscience, 1977) pp. 424 to 467 (chapter12 “Preparation of 6,6-Nylon and related polyamides” by Donald B. Jacobsand Joseph Zimmerman) comprise, in the finishing step of the polymer, aflasher, a vapour/prepolymer separator and a finisher. The flow ofprepolymer obtained by the process of the invention is added to theconventional flow of polyamide upstream of the flasher.

[0127] The addition of this flow of polyamide leaving the process inaccordance with the invention does not disrupt the quality or propertiesof the polyamide obtained leaving the finishing steps. These propertiesare identical to those of the polyamide manufactured without thisadditional flow.

[0128] The process of the invention thus makes it possible tomanufacture a compatible polyamide to be used as a starting material inthe usual uses, such as the manufacture of yarns, fibres or films or themanufacture of moulded articles.

[0129] The on-line Near InfraRed spectrometric analysis is now describedin detail.

[0130] The method of measurement by spectrometric analysis in the nearinfrared spectral region consists in performing a continuous measurementby transmission on the reaction mixture: the light wave is emitted bythe spectrometer lamp, conveyed by a single strand of optical fibre tothe emitting probe which is directly in contact with the reactionmixture. The light information is partially absorbed by the productpassed through and is then captured by the receiving probe which isrigorously aligned with the emitting probe, conveyed via a second singlestrand of optical fibre and then collected by the spectrometer detector.The spectral acquisition software constitutes the whole transmissionspectrum to convert it into an absorbance spectrum. The acquisition ofthe spectra thus takes place over a range of wave numbers from 4 600 to9 000 cm⁻¹ with a resolution of 16 cm⁻¹: each spectrum results from theaverage of 32 scans formed at an average speed of 128 scans per minute.

[0131] The spectral information collected by continuous analysis istranscribed into a concentration of acid and amine end groups per kg ofdry product (CEG and AEG, respectively) and also into a CEG-AEGdifference using models produced by calibration with samples analysed bypotentiometric analysis methods described, for example, in “Encyclopediaof Industrial Chemical Analysis”, 1973, volume 17, page 293.

[0132] The assembly of the device for near infrared measurement is ratedso as to withstand an internal pressure of 150 bar and a temperature of300° C. The assembly is composed of an APX4 stainless steel cellprovided by the company Ateliers Mecaniques Peagois (AMEP), Peage deRoussillon, France.

[0133] The cell body is heated electrically, the heating beingcontrolled by a temperature measurement in the metal bulk and in thepolymer. An alarm on the temperature difference makes it possible todetect a fault in the heating device.

[0134] The pipe for circulating the analysed product is cylindrical and1 cm in diameter. This flow is intercepted perpendicularly by the probesdirectly screwed onto the body of the cell.

[0135] The probes used are of the type FCP-080 cross line probe suppliedby Axiom Analytical Incorporated, Irvine, Calif. These probes arescrewed into the probe holders so as to produce a conical metal-metalseal, the sapphire of about 8 mm being flush with the end of the probeholder. The spacing between the emitting probe and the receiving probeis thus made by a symmetrical adjustment by screwing the two probeholders face to face: it is set at 4 mm and remains constant during thecalibration and prediction phases in continuous use.

[0136] The probes are connected to the spectrometer via an optical fibremeasuring up to 50 metres. The spectrometer itself is connected to acomputer in an operating room which gives a real-time report of theresult of the on-line analysis.

[0137] The measurement performed on the mixtures M2 and M3 at the outletof the reactors 104 and 105 shows a standard error of prediction of 10.1meq/kg for AEG, 13.0 meq/kg for CEG and 12.7 meq/kg for the differenceCEG-AEG with correlation coefficients of greater than 0.99.

[0138] The degree of precision achieved by this statistical analysis ofthe near infrared spectra allows an adjustment of the ratio between theacid end groups and amine end groups in the mixtures M2 and M3 bycoupling the flow rates of the fluids fed into the reactors 104 and 105,respectively.

[0139] The measurement formed at the outlet of the reactor 107 shows astandard error of prediction of 4.6 meq/kg for AEG, 5.1 meq/kg for CEGand 4.7 meq/kg for the difference CEG-AEG with correlation coefficientsof 0.990 for AEG, 0.991 for CEG and 0.995 for the difference CEG-AEG.

[0140] The degree of precision achieved by this statistical analysis ofthe near infrared spectra also allows an adjustment of the ratio betweenacid end groups and amine end groups by coupling the flows of themixtures M2 and M3 fed into the premixer or the reactor 107.

[0141] Similarly, this analysis method by near infrared spectrometryalso makes it possible to determine the composition of the mixture Mland to couple the flow rates of the monomers fed into the reactor 101.

[0142] The placing of the probes in the process may be differentdepending on the arrangement of the reactors or the presence of storageunits or reactors.

[0143] The number of analysis points on the process may vary from one toseveral. Thus, it is possible to have only one monitoring of thecomposition of the prepolymer leaving the reactor 107 and to couple thismeasurement to the flow rates of the mixtures M2 and M3 and/or the feedrates of the HMO solution and of the mixture M1 in the reactors 104 and105, without, however, departing from the context of the invention.However, for better monitoring of the process, it is preferable tomonitor the composition of each of the mixtures M1 to M4 and to couplethe feed rates of the reagents into each reactor for manufacturing thesemixtures.

[0144] One example of a system for monitoring and coupling the feedrates of the flows of mixtures into the various reactors is illustratedby the dashed lines in FIG. 1.

[0145] As mentioned above, the process of the invention preferentiallyapplies to the manufacture of PA 6,6, but it may also be used for themanufacture of other polyamides obtained from diacid and diaminemonomers, and especially for the manufacture of copolyamides, forinstance copolyamides PA 6,6/6.

[0146] The starting operations and the transient states of the productsand process parameters before obtaining the permanent regime describedabove are now described.

[0147] The start-up uses the N salt at a concentration of 62%. It isprepared from a source of water 254, adipic acid 252 andhexamethylenediamine 251.

[0148] In the example illustrated, the detailed description of which isgiven with reference to FIG. 2 which represents a synoptic diagram of apart of the process of the invention, the solution of N salt at aconcentration of 62% is manufactured in the workshop 224. This workshopis fed continuously via at least two buffer tanks of the type 215 and216 which empty alternately by respiration between a maximum level and aminimum level. These nitrogen-inertized tanks are equipped with a meansof stirring by external recycling so as to homogenize the contenttherein and to monitor the pH and the concentration therein beforefeeding the polycondensation unit.

[0149] State 1: Filling of the Plants

[0150] After the usual water tests preceding the start-up of thepolycondensation plant, the water feed is replaced with a feed of a 62%N salt solution introduced continuously at 109° C., at a flow rate of280 I/h (≅300 kg/h), into the reactor 201 and then into the assembly ofthe cascade of stirred reactors 204, 205 and 208. The outlet valves ofthe various reactors are closed in a first stage, to allow them to fill,and then opened. The plants comprise means for adjusting the levels. Theflow of exiting solution is reintroduced into the plants. The reactorsfunction at the feed temperature of the 62% N salt under a virtuallyatmospheric autogenous pressure of 1.05 bar to avoid the introduction ofoxygen.

[0151] Stage 2: Establishment of the Operating Regime in the Plants inthe Future State of the Reactor 201 in the Permanent Regime

[0152] After stabilizing the levels during the preceding step, thecontinuous feed rate of 62% N salt is reduced to 110 I/h (˜120 kg/h) soas to allow the concentration of the 62% N salt to [lacuna] 75% with theaid of heat-exchange surfaces arranged in each of the stirred reactors.To do this, the nominal temperatures in the reactors 201, 204, 205 and207 are set at 115° C. The corresponding autogenous pressure is thenadjusted to 1.05 bar.

[0153] State 3: Establishment of the Operating Regime in the Reactors204, 205 and 207 in the Future State of the Reactors 204 and 205 in thePermanent Regime

[0154] After stabilizing the levels and temperatures during thepreceding step, the nominal temperatures are adjusted to 180° C. in thereactors 204, 205 and 207.

[0155] The autogenous pressure is then adjusted to 5 bar. Theconcentration of the N salt in the reactor 207 is 88%. This step isimmediately linked into the following step as soon as the temperature of180° C. is reached in the reactor 207.

[0156] State 4: Establishment of the Operating Regime in the Reactor 207in the Final State of the Reactors 204 and 205 in the Permanent Regime

[0157] Once the nominal temperature of 180° C. has been reached in thereactor 207, this temperature is adjusted to 227° C. The autogenouspressure is then adjusted to 15 bar.

[0158] State 5: Establishment of the Operating Regime in the Reactors204, 205 and 207 in Their Final State (Fed With N Salt Solution)

[0159] The nominal temperatures are adjusted to 228, 226 and 248° C.,respectively, for the reactors 204, 205 and 207. The autogenouspressures are then adjusted to 15 bar in the reactors 204 and 205 and17.5 bar in the reactor 207. The number-average molar mass measured on adry extract of the flow leaving the reactor 207 is about 4 000 g/mol.This flow continuously feeds the downstream of the polycondensationprocess consisting of a combination in series of a flasher, avapour/prepolymer

[0160] separator and a finisher. The polymer obtained at the finisheroutlet has a viscosity index measured in dilute formic acid inaccordance with the conditions and recommendations of the internationalstandard ISO 307-1984 of 135 ml/g.

[0161] The product leaving the cascade of stirred reactors is theprepolymer which gives, after rising in bulk, a polymer whose viscosityindex is suitable for most conversion applications, either directly onleaving the finisher, or after granulation followed by remelting.

[0162] This step is advantageously used to calibrate the points formeasuring the stoichiometry by near infrared, such as a descriptionthereof is given in Example 2.

[0163] State 6: Final State Corresponding to the Permanent Regime

[0164] When all of the near infrared measuring points have beencalibrated and when the spectrometer has thus been calibrated, in aprecise and reliable manner to allow the direct amidation process to beconducted, the feed of 62% N salt into the reactor 1 is replaced with acontinuous feed at 90 kg/h of the mixture M1 described above.

[0165] The adjustment of the stoichiometry using the on-line nearinfrared measurements is activated on the assembly of cascade reactors:the coupling between the adjustment of the stoichiometry of the exitflow and the adjustment of the level in each of the reactors 201, 204,205 and 207 allows the flow rates of the two feed flows of each of thereactors to be adjusted, one of these flows being systematically rich inamine functions and the other rich in acid functions.

[0166] This mode of start-up of the direct amidation process using anaqueous solution of monomers makes it possible to achieve the permanentregime described at the start of this Example 1 by allowing thecalibration of the points for measuring the stoichiometry by nearinfrared and by minimizing, in the calibration and standardizationphases, the risks of producing non-specification polymer.

EXAMPLE 2 Example of a Procedure for Calibrating a Near InfraredSpectrometer to Carry out the Process.

[0167] Devices and procedures relating to the near infrared analysis andthe calibration of the spectrometer are described in this example.

[0168] First Branch Loop

[0169] Although the measurement can be carried out on the main productcirculation pipe, the measurement is carried out on a branch loop.

[0170] Second Branch Loop

[0171] A second, closed, branch loop connected to the first loop isused. This loop makes it possible to standardize and calibrate the nearinfrared spectrometer.

[0172] Calibration Procedure

[0173] The product obtained from the condensation using the N salt, thecharacteristics of which are known, is recovered in the second branchloop. With the second branch loop connected to the spectrometer, thespectrometer is calibrated in the desired range using referencemeasurements produced from samples taken from the loop, followingvarious dosed additions of known amounts of adipic acid and/or ofhexamethylenediamine (dosed addition method) in the loop.

EXAMPLE 3

[0174] Another embodiment of a process according to the invention.

[0175] The plants and the functioning in a permanent regime are firstdescribed.

[0176] According to the scheme represented in FIG. 3, into a zoneconsisting of a combination in series of static mixers 403 arecontinuously fed, via the pipes 401 and 402, respectively, a flow ofhexamethylenediamine melt obtained from a source 451, and a flow ofdeoxygenated adipic acid melt at a temperature of 250° C. under apressure of about 15 bar, obtained from a source 452. The mixtureobtained is referenced M5. The stoichiometry of the reaction mixture ismeasured continuously by near infrared in the flow 407 leaving themixing zone 403: the flow rates of the flows 401 and 402 are thenadjusted so as to regulate the nominal value of the molar ratio of thenear infrared measuring point 408.

[0177] The mixture M5 is introduced into the recycling loop of a stirredreactor 405: this loop is equipped with a heat exchanger 406 andprovides via the pump 410 a recycling flow rate which is 20 times asgreat as the feed rate of the mixture M5. The reactor 405 operates at atemperature of 255° C. under a pressure adjusted to 15 bar. It isequipped with an external device for partial recovery of the losses ofvolatile diamine entrained by the steam, not represented in FIG. 3. Thefilling level is such that the estimated residence time is about 20minutes.

[0178] A fraction of the flow leaving the reactor 405 is removed via thepipe 411. A continuous near infrared measurement point located in thispipe 411 makes it possible to monitor the stoichiometry and the chemicalprogress of the product. If need be, it assigns a new nominal value tothe near infrared measurement point 408 located on the pipe 407.

[0179] The prepolymer 453 thus produced is converted into polyamide ofdesired molar mass which is compatible with the usual uses by additionto a polyamide manufacturing flow obtained from a process formanufacturing polyamide 6,6 using hexamethylenediamine adipate salt.

[0180] The continuous and usual processes for manufacturing PA 6,6described, for example, in the book “Polymerization processes”,Schildknecht edition (Wiley Interscience, 1977), pp. 424 to 467 (chapter12, “Preparation of 6,6-Nylon and related polyamides” by Donald B.Jacobs and Joseph Zimmerman) comprise in the polymer finishing step, aflasher, a vapour/prepolymer separator and a finisher. The flow ofprepolymer obtained by the process of the invention is added to theconventional flow of polyamide upstream of the flasher.

[0181] An alternative may also consist in passing the reaction mixturethrough a flasher, a finisher classified as being of low viscosity, fromwhich the prepolymer leaves with a viscosity index, measured in diluteformic acid in accordance with the conditions and recommendations ofinternational standard ISO 307-1977 in the region of 60 ml/g, so as tobe able to be granulated. A drying operation accompanying apost-condensation in solid form then makes it possible to achieve therequired level of viscosity for the polymer.

[0182] The method of measurement by spectrometric analysis in the nearinfrared spectral region is performed in a manner which is entirelysimilar to the description in Example 1. This on-line measurement iscarried out at all the relevant points of the plant and moreparticularly at points 408 and 409 represented in FIG. 3.

[0183] The start-up operations and the transient states of the productsand process parameters before obtaining the permanent regime describedabove are now described.

[0184] The start-up uses N salt at a concentration of 62%. It isprepared from a source of water 554, adipic acid 552 andhexamethylenediamine 551.

[0185] In the example illustrated, the detailed description of which isgiven in reference to FIG. 4 which represents a synoptic diagram of onepart of the process of the invention, the solution of N salt at aconcentration of 62% is manufactured in the workshop 521. This workshopis fed continuously via at least two buffer tanks of the type 515 and516 which empty alternately by respiration between a maximum level and aminimum level. These nitrogen-inertized tanks are equipped with a modeof stirring by external recycling to homogenize the content and tomonitor the pH and the concentration therein before feeding into thepolycondensation unit.

[0186] State 1: Filling of the Plants

[0187] After the usual water tests preceding the start-up of thepolycondensation plant, the water feed is replaced with a feed of a 62%N salt solution introduced at 109° C., into one of the two stirredreactors 513 and 514. These two reactors are equipped with a recyclingloop consisting of a pump, a heat exchanger and a near infraredmeasurement point. Coupled to the buffer tanks 515 and 516, they operatealternately so as to continuously feed the stirred reactor 515 via itsrecycling loop 504.

[0188] State 2: Establishment of the Operating Regime in the Reactors513 and 514 Under the Conditions of the Permanent Regime

[0189] After stabilizing the levels during the preceding step, thenominal temperatures in the reactors 513, 514 and 505 are set at 180° C.The corresponding autogenous pressure is then adjusted to 6 barabsolute.

[0190] The concentration of the N salt leaving the reactor 505 is 90%.

[0191] State 3: Establishment of the Operating Regime in the Reactor 505Under the Conditions of the Permanent Regime (Feeding With N SaltSolution)

[0192] Once the nominal temperature of 180° C. has been reached in thereactor 505, this temperature is adjusted to 260° C. therein and thelevels are adjusted so as to obtain an estimated residence time of about40 minutes. The autogenous pressure is then adjusted to 15 bar. Thenumber-average molar mass measured on a dry extract is about 3 800g/mol. This flow continuously feeds the downstream of thepolycondensation process consisting of a combination in series of aflasher, a vapour/prepolymer separator and a finisher. The polymerobtained at the finisher outlet has a viscosity index, measured indilute formic acid in accordance with the conditions and recommendationsof international standard ISO 307-1984, of 130 ml/g.

[0193] The product leaving the cascade of stirred reactors is theprepolymer which gives, after increasing in mass, a polymer with aviscosity index that is suitable for most of the conversionapplications, either directly to the finisher outlet, or aftergranulation followed by remelting.

[0194] This step may advantageously be used to calibrate the points formeasuring the stoichiometry by near infrared as a description thereof isgiven in Example 2.

[0195] State 4: Final State Corresponding to the Permanent Regime(Feeding With Monomer Melts)

[0196] When all of the near infrared measurement points have beencalibrated, and the spectrometer has thus been calibrated, in a preciseand reliable manner to allow the process in permanent regime to beconducted, the feed of 62% N salt into the reactor of type 513 or 514 isreplaced with a continuous feed of the mixture M5 as describedpreviously into the mixing zone 503. The residence time is adjusted toabout 20 minutes.

[0197] The adjustment of the stoichiometry from the on-line measurementsby near infrared is activated: the coupling between the adjustment ofthe stoichiometry of the flow leaving the reactor 505 and its leveladjustment makes it possible to adjust the flow rates of the two feedflows 501 and 502.

[0198] This start-up mode of the direct amidation process using anaqueous monomer solution makes it possible to achieve the permanentregime described at the start of this Example 3 by allowing thecalibration of the points for measuring the stoichiometry by nearinfrared and by minimizing, in the calibration phase, the risks ofproduction of non-specification polymer.

[0199] For the measurements by near infrared spectrometry and thecalibration, the primary and secondary branch device described inExample 2 is used.

[0200] This calibration procedure may be used judiciously at any of themeasurement points 508, 509, 517, 518, 519 or 520 mentioned in FIG. 4for the on-line monitoring of the stoichiometry, or even of the watercontent, irrespective of the degree of progress of the polycondensation,from the monomers and the N salt up to the prepolymer leaving the finalreactor and, further downstream, to the polymer leaving the finisher.

1. Process for the continuous manufacture of polyamide, starting withdicarboxylic acids and diamines, comprising, in a permanent regime, thefollowing steps: continuous mixing of two flows of polyamide precursors,one flow rich in acid end groups, and one flow rich in amine end groups,condensation using the mixture of the two flows, the mixing andcondensation being carried out in the same plant or in different plants,recovery of a flow of product obtained from the condensation operation,at least one of the flow rates of the flows of precursor being coupledto the result of the analysis of the contents of acid and amine endgroups in the mixture, the condensation medium or the recovered flow, soas to keep these contents and/or their ratio between two nominal values,characterized in that the starting phase comprises the following steps:Step 1: feeding the condensation plant with an aqueous solutioncomprising dicarboxylate ions and diammonium ions, in proportions suchthat their molar ratio is between 0.99 and 1.01 and preferably between0.995 and 1.005, establishment of the operating regime in thecondensation plant under temperature and pressure conditions such thatthere is condensation starting with the dicarboxylate ions and thediammonium ions initially included in the aqueous solution, Step 2:recovery of a flow of the condensation product Step 3: supplying a flowof precursors and activating the analysis and the coupling.
 2. Processaccording to claim 1, characterized in that the flow of recoveredproduct is fed into flashing and/or finishing plants so as to obtain apolyamide with a desired degree of polymerization.
 3. Process accordingto either of claims 1 and 2, characterized in that the condensationusing the aqueous solution is performed at a temperature above 180° C.,at a pressure above 5 bar, the product recovered having a degree ofpolycondensation of greater than 10, preferably of greater than 20, andin that the temperature and pressure are identical to the temperatureand pressure in the permanent regime.
 4. Process according to either ofclaims 1 and 2, characterized in that the analysis is performed by NearInfraRed spectrometry.
 5. Process according to claim 4, characterized inthat at least some of the flow of the condensation product from thesolution is recovered and used to calibrate the Near InfraRedspectrometer in the desired range of contents of end groups and/or ofratios of contents, the feeding with two flows, the analysis and thecoupling being activated after this operation.
 6. Process according toclaim 4, characterized in that the calibration is carried out by themethod of dosed additions into the part of the flow recovered for thispurpose.
 7. Process according to one of the preceding claims,characterized in that the diacid monomers are chosen from the listcomprising adipic acid, glutaric acid, suberic acid, sebacic acid,dodecanedioic acid, isophthalic acid, terephthalic acid, azelaic acidand pimelic acid.
 8. Process according to one of the preceding claims,characterized in that the diamine monomers are chosen from the listcomprising hexamethylenediamine, heptamethylenediamine,octamethylenediamine, nonamethylenediamine, decamethylenediamine,5-methylpentamethylenediamine, undecamethylenediamine,dodecamethylenediamine and xylylenediamine.
 9. Process according to oneof the preceding claims, characterized in that the diacid monomercomprises at least 80 mol % of adipic acid.
 10. Process according to oneof the preceding claims, characterized in that the diamine monomercomprises at least 80 mol % of hexamethylenediamine.
 11. Processaccording to one of the preceding claims, characterized in that: theflow which is rich in acid end groups is a mixture M2 of diacid and ofdiamine with a diacid/diamine molar ratio of between 1.005 and 1.2 andpreferably between 1.01 and 1.2 the flow which is rich in amine endgroups is a mixture M3 of diacid and of diamine with a diacid/diaminemolar ratio of between 0.8 and 0.995 and preferably between 0.8 and 0.99the two flows are fed in melt form into a stirred polymerization reactorin which the condensation is carried out the condensation product is aprepolymer fed into flashing and/or finishing steps to obtain thedesired degree of polymerization.
 12. Process according to claim 11,characterized in that the two flows are mixed together before feedinginto the condensation plant.
 13. Process according to either of claims11 and 12, characterized in that the polymerization reactor operatesunder an autogenous pressure or an adjusted pressure.
 14. Processaccording to one of claims 11 to 13, characterized in that theprecursors M2 and M3 are obtained, in a permanent regime, by mixing asolution of diamine monomer with a mixture Ml which is rich in diacidmonomer.
 15. Process according to claim 14, characterized in that atleast one of the flow rates of the flows for introducing diamine monomersolution and mixture M1 which is rich in diacid monomer is coupled tothe result of the continuous analysis of the acid and amine functions inthe precursors M2 and M3.
 16. Process according to one of claims 11 to15, characterized in that the precursors are prepared in a permanentregime by mixing a flow of hexamethylenediamine with an adipicacid/hexamethylenediamine eutectic mixture.
 17. Process according toclaim 15 or 16, characterized in that the precursors are prepared by aprocess comprising the following steps: feeding each of the precursorpreparation plants with an aqueous solution comprising dicarboxylateions and diammonium ions in proportions such that their molar ratio isbetween 0.99 and 1.01 and preferably between 0.995 and 1.005establishing the operating regime in the precursor preparation plantsunder the temperature and pressure conditions of the permanent regime ineach of the precursor preparation plants, feeding a flow of mixture Mlwhich is rich in diacid monomer, a flow of hexamethylenediamine,activation of the coupling of at least one of the flows, feeding theproduct obtained into the mixing or condensation plant.
 18. Processaccording to one of claims 15 to 17, characterized in that the eutecticmixture M1 is prepared by mixing a flow of adipic acid melt or solid,and a flow of hexamethylenediamine in pure form or in the form of aconcentrated aqueous solution, with coupling of at least one of theflows to the result of the analysis of the content of acid and aminegroups in the mixture Ml, the process comprising the following steps:feeding the plant for preparing the eutectic mixture with an aqueoussolution comprising dicarboxylate ions and diammonium ions, inproportions such that their molar ratio is between 0.99 and 1.01 andpreferably between 0.995 and 1.005 establishing the operating regimeunder the temperature and pressure conditions of the permanent regimefeeding a flow of adipic acid, a flow of hexamethylenediamine,activation of the coupling of at least one of the flows, feeding theproduct obtained into the precursor preparation plants.
 19. Processaccording to one of claims 1 to 10, characterized in that: the flowwhich is rich in acid end groups is a dicarboxylic acid in melt form,the flow which is rich in amine end groups is a diamine in pure form orin the form of a concentrated aqueous solution.
 20. Process according toclaim 18, characterized in that the diacid is adipic acid and thediamine is hexamethylenediamine.